Let A be the area of a circle with radius r. if dr/dt=2, find dA/dt when r=5

Answer 1

A=pi r^2

dA/dr=2.pi.r

dA/dt=dA/dr * dr/dt

dA/dt=2 * pi * r * 2

dA/dt=2 * pi * 5 *2

=20 pi

Answer 2

A = pi*R^2

dA/dt = 2 * pi * R * dR/dt

= 2 * pi * 5 * 2

= 20 * pi